1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for producing the same.
2. Description of the Related Art
Conventionally, solid electrolytic capacitors having a small equivalent series resistance (ESR) have been known (see, for example, Japanese Unexamined Patent Publication Nos. 4-48710, 10-92699 and 10-321471).
FIG. 9 is a sectional view for explaining the structure of a conventional solid electrolytic capacitor. The structure of this conventional solid electrolytic capacitor will be explained with reference to FIG. 9.
In the conventional solid electrolytic capacitor, as shown in FIG. 9, a capacitor element 102 is buried in a rectangular parallelepiped-like outer package 101.
The capacitor element 102 is provided with an anode 104 in which a part of an anode lead 103 is buried and which is made of a porous sintered body of tantalum particles, a dielectric layer 105 containing a tantalum oxide formed on the anode 104 and a cathode 106 formed on the dielectric layer 105.
The anode lead 103 is constituted of tantalum, wherein one end of the anode lead 103 is buried in the center of the anode 104 and one end of an anode terminal 107 is connected to the other end of the anode lead 103 exposed from the anode 104. Also, the other end of the anode terminal 107 is exposed from the outer package 101.
The cathode 106 comprises a first electrolyte layer 106a made of polyethylenedioxythiophene (PEDOT) formed on the dielectric layer 105, a second electrolyte layer 106c which is made of polypyrrole (PPy) and formed on the first electrolyte layer 106a, a first conductive layer 106d which is formed on the second electrolyte layer 106c and contains carbon particles and a second conductive layer 106e which is formed on the first conductive layer 106d and contains silver particles.
One end of a cathode terminal 109 is connected to the surface of the cathode 106 through a third conductive layer 108 containing silver particles and the other end of the cathode terminal 109 is exposed from the outer package 101. The conventional solid electrolytic capacitor is thus constituted.
Next, a process for producing the conventional solid electrolytic capacitor will be explained with reference to FIG. 9.
In the case of the conventional solid electrolytic capacitor, first, a rectangular parallelepiped-like molded article is formed from tantalum particles and also, one end of the anode lead 103 is buried in this molded article. Next, this molded article is sintered under vacuum to thereby form the anode 104 in which a part of the anode lead 103 is buried.
Then, the anode 104 is subjected to anodic oxidation carried out in an aqueous phosphoric acid solution to form the dielectric layer 105 made of a tantalum oxide on the surface of the anode 104.
Next, the first electrolyte layer 106a made of PEDOT is formed on the dielectric layer 105 by chemical polymerization of ethylenedioxythiophene (EDOT). Moreover, the second electrolyte layer 106c made of PPy is formed on the first electrolyte layer 106a by electrolytic polymerization of pyrrole (Py). Also, a carbon paste containing carbon particles is applied to the second electrolyte layer 106c and dried to thereby form the first conductive layer 106d containing carbon particles on the second electrolyte layer 106c. Further, a silver paste containing silver particles is applied to the first conductive layer 106d and dried to form the second conductive layer 106e containing silver particles on the first conductive layer 106d. The capacitor element 102 provided with the cathode 106 including these layers 106a, 106c, 106d and 106e on the dielectric layer 105 is thus formed.
Next, the anode terminal 107 is connected to the anode lead 103 exposed from the anode 104 by welding. Also, the cathode 106 and the cathode terminal 109 are dried in the condition that the both are stuck to each other through a silver paste containing silver particles to thereby form the third conductive layer 108 containing silver particles between the cathode 106 and the cathode terminal 109 and also, the cathode 106 is connected to the cathode terminal 109 through the third conductive layer 108.
Finally, the capacitor element 102 to which the anode terminal 107 and the cathode terminal 109 are connected is buried in a resin composition and this resin composition is heat-cured to form the outer package 101 provided with the buried capacitor element 102. The conventional solid electrolytic capacitor is manufactured by the above method.